Paper webs, such as those used in disposable consumer tissue products (facial tissue, bathroom tissue, towels, etc.) have been traditionally made by the four-drinier process, which involves laying an aqueous solution of fibers and water on a moving foraminous forming fabric, dewatering the web thus formed with a combination of vacuums, heat and pressure, and creping the substantially dry web. Dewatering the web produces stiff hydrogen bonds between adjacent fibers and lends strength to the web, while a subsequent creping process breaks a portion of these bonds and produces favorable tactile properties. Because the stock slurry contains only approximately 0.4% fibers, substantial amounts of energy must be expended in order to remove the water. Typically, the web is dewatered to approximately 15 to 20% fiber consistency while on the forming fabric, primarily by the application of vacuum to the web, with subsequent dewatering taking place between press rolls which bring the fiber consistency up to approximately 40%. Final dewatering may occur on a Yankee drying cylinder which dries the web to approximately 95% consistency. Alternatively, in place of the press rolls and Yankee dryer, a throughdrying cylinder as shown in U.S. Pat. No. 3,303,576, Sisson may be employed, which substantially dries the web prior to its application to a creping cylinder. Because the energy input (in the form of heat) required to remove a given quantity of water from a wet web increases dramatically as the web becomes drier, any method of reducing the amount of water in the sheet which would decrease the drying load would be beneficial in terms of reducing the cost of manufacture for a given quantity of paper webs.
As has been proposed previously, such as in U.S. Pat. No. 4,166,001, Dunning et al., numerous benefits may be derived from multi-layer web exhibiting ply separation, such as increased bulk and absorbency of the finished product. In the Dunning et al patent, it was suggested that forming a three layer web with the two outer layers forming strong inter-fiber bonds and the inner layer being of fibers forming weak inter-fiber bonds would aid in ply separation. The difference in bonding occurred through use of different fiber types, such as using northern softwood fibers with wet strength resin in the outer layers and hardwood fibers with chemical debonders in the inner layer.
It has been proposed in U.S. Pat. No. 3,954,554, Curry et al., to form a multi-ply paper sheet or paperboard by using multiple forming means to form separate plies which are subsequently brought together to form a single product. For example, a fourdrinier-type forming unit forms a first wet-laid web, multiple airlaying forming heads form a second air-laid web which is brought into contact with the first wet-laid web, and a third fourdrinier forming unit forms a second wet-laid web which is thereafter brought into contact with the air-laid web to form a three layer product. It is suggested that the reduced drying load of the machine will permit it to be operated much faster, thereby increasing the efficiency of the process.